1. Field of the Invention
The present invention relates to a chromium-free rust inhibitive treatment method for metal products having a zinc surface such as galvanized products and metal products having a zinc surface, which are treated by the chromium-free rust inhibitive treatment. The zinc surface for the invention can be an alloyed zinc surface.
2. Description of the Related Art
Conversion coating is a process for forming stable compound layer on metal surfaces by chemical or electrochemical treatment, and JIS standard books describe that there are phosphate treatment, blackening treatment, chromate treatment, etc. The main purposes of the conversion coating of metal products are to improve their rust inhibitive performance, to improve their applicability of paint and to color their surfaces.
Conventionally, metal products having zinc surfaces, for example, surfaces of galvanized metal products are treated by conversion coating using an aqueous solution containing chromate, or conversion coating using an aqueous solution not containing chromate. Methods for imparting rust inhibitive performance by further coating the metal surfaces with a paint film containing an organic resin as a base component have come into widespread use.
Because of the toxicity and carcinogenicity of hexavalent chromium, policies of not using products containing hexavalent chromium have been first formulated in Europe and the development of surface treatment agents not containing chromic acid (chromate) component is being actively carried out. Although some chromium-free surface treatment methods have already been proposed in laid-open patent documents, at present, galvanized products treated by conversion coating using chromite (trivalent chromium) for rust inhibition have just entered the stage of practical use. However, in performing the chromite conversion coating, liquid control is not easy and the chromite conversion coating poses problems, because part of trivalent chromium component changes into hexavalent chromium, which must be excluded from waste water.
There has also been proposed a method which involves treating a metal product having a zinc surface, for example, a surface of a galvanized metal product by conversion coating and further forming a film on the surface with an aqueous surface treatment agent containing silica and others to improve rust inhibitive performance.
For example, the Japanese Patent Laid-Open No. 61-253381 discloses a method to improve rust inhibitive performance which involves blackening a galvanized surface by conversion coating, followed by coating the conversion coated surface by applying an aqueous solution of silicate or an aqueous solution of colloidal silica etc., and an improved rust inhibitive performance was able to be imparted. However, in galvanized steel sheets subjected to rust inhibitive treatment by the method, white rusting occurred in less than 48 hours in the salt spray test specified in the Japan Industrial Standard (JIS) and the rust inhibitive performance of the galvanized steel sheets was insufficient in practical use.
The Japanese Patent Laid-Open No. 2000-248367 discloses a case where a surface of a zinc-plated steel sheet was treated by phosphate conversion coating and a thin film was formed on the treated surface by use of an aqueous solution which contained an organic resin as a base component and also contained a thiocarbonyl-group-containing compound, a phosphate compound, fine-grained silica and a silane coupling agent in addition to the organic resin. Alternatively, a thin film was formed on the conversion coated surface by use of an aqueous solution which contained a vanadic acid compound, a thiocarbonyl-group-containing compound, a phosphate compound, fine-grained silica and a silane coupling agent in addition to the organic resin as a base component. The patent document describes that there was no change even after samples of the surface-treated galvanized steel sheet were kept in a salt spray test apparatus for 168 hours.
The Japanese Patent Laid-Open No. 2001-81578 discloses a pre-painted metal sheet which was obtained by treating a galvanized steel sheet with chromium-free conversion coating, applying a polyester-based primer paint (thickness: 4 to 25 μm) containing a chromium-free rust inhibitive pigment to the surface of the galvanized steel sheet and applying a topcoat paint, and exemplifies modified silica (Sealdex: a fine powder of silica made by Fuji Silysia Chemical Ltd.) as the chromium-free rust inhibitive pigment.
The Japanese Patent Laid-Open No. 2002-317279 discloses a steel sheet which was obtained by conversion coating a hot-dip alloy galvanized steel sheet (containing Al and Mg), and applying undercoat with an aqueous paint and further applying a topcoat. The conversion coating liquid used contained a titanium compound, such as hexafluorotitanic acid and titanium hydrofluoric acid, and a zirconia compound, such as hexafluorozirconic acid in addition to an aqueous resin, such as an acryl emulsion, and the patent document exemplifies an aqueous paint, as a primer paint, which contained an aqueous acryl emulsion and in which a silica-based rust inhibitive pigment (Sealdex) was dispersed. Then, an aqueous paint which contained an aqueous resin as a base component, such as an acryl emulsion, and in which a titanium dioxide pigment was dispersed, was applied as a topcoat paint. From examples of the patent document, rust inhibitive properties of primer painted sample sheets are not clear, since the evaluation was performed by the blistering of the coated films when the topcoated sample sheets were put in a salt spray test apparatus.
Also, the Japanese Patent Laid-Open No. 2003-253464 discloses a steel sheet which was obtained by treating commercially available galvanized steel sheets (an electrogalvanized steel sheet and a hot-dip galvanized steel sheet, these commercially available steel sheets are usually subjected to chromate treatment) with zinc phosphate type conversion coating, and applying a chromium-free film coating on the surfaces. The chromium-free film coating was formed by a slightly acidic aqueous solution containing a phenol-based aqueous organic resin, a titanium compound, such as titanium ammonium fluoride, or a zirconium compound, such as zirconium hydrofluoric acid, and a silane coupling agent, such as mercaptopropyl trimethoxylane. The patent document describes that when samples of the surface-treated galvanized steel sheet were put in a salt spray test apparatus for 72 hours, some samples did not cause white rusting.
The Japanese Patent Laid-Open No. 05-001391 discloses that rust inhibitive performance (white rusting resistance) was improved by forming a film of a surface treatment agent containing alcohol and silica on a conversion coated galvanized surface. However, the film formed by applying the surface treatment agent contained a resin as a base component and it is hard to say that the film was siliceous. According to examples described in the patent document, all the films contained resin components including a curing agent resin such as melamine resin in amounts of not less than 51% by weight. Furthermore, the patent document describes only examples in which a surface treatment agent was applied to a galvanized and chromate treated surface.
The Japanese Patent Laid-Open No. 2001-64782 discloses that silica sol in ethanol solvent, which was obtained from ethanol solution of tetraethoxysilane through hydrolysis with water and hydrochloride acid, was used as a surface treatment agent. A hot-dip zinc alloy galvanized steel sheet, which was subjected to colorable conversion treatment before, was dipped in the surface treatment agent to form a siliceous coating on the steel sheet and then the coating was cured. The rust inhibitive performance of the surface treated steel sheet in a salt spray test is described in the patent document, that white rusting appeared on a sample of the surface treated steel sheet in 12 hours. The reason of the poor rust inhibitive performance is supposed that the surface treated steel sheet had a siliceous coating thinner than 0.3 μm since the used silica sol solution had silica component concentration less than 5% by weight.
The present inventors proposed in their previous patent application (the United States Patent Application Publication No. US 2005/0037227 A1) a chromium-free surface treatment agent which forms a siliceous film capable of suppressing the formation of red rusting for a long time when applied to a surface of a galvanized metal product. In the chromium-free surface treatment agent, a nano-sized powder of titanium dioxide subjected to dispersion treatment, which has an average particle size of primary particles of not more than 70 nm (preferably not more than 40 nm), was mixed in an effective amount.
Also, in the US Patent Application Publication, the present inventors proposed a surface treatment agent effective in suppressing the formation of white rusting. The surface treatment agent was an alcoholic solution which contained alkoxysilane oligomer having a specific weight averaged molecular weight as a base component.
That is, not only the formation of red rusting, but also the formation of white rusting can be suppressed for a long time by applying the surface treatment agent, which is an alcoholic solution containing alkoxysilane oligomer having a weight averaged molecular weight of 1000 to 10000 as a base component, to a surface of a galvanized product to coat the surface with a thin siliceous film of 1 to 3 in thickness. In the case, when the affinity of the surface treatment agent with the zinc plated condition of the substrate is good, the formation of white rusting can be suppressed for 300 hours or longer in the salt spray test.
It is preferred that non-VOC water be used as a solvent of the surface treatment agent. However, when a chromium-free surface treatment agent which uses only water as a solvent was applied to a surface of a galvanized product, the formation of white rusting occurred early even if the formation of red rusting can be suppressed for a long time (as described in the Japanese Patent Laid-Open No. 61-253381).
When the present inventors obtained from various manufacturers bolts which had been galvanized under various galvanizing conditions (without chromate treatment) and applied a surface treatment agent which was an alcoholic solution containing alkoxysilane oligomer as a base component, it was found that rust inhibitive performance varies greatly depending on galvanizing conditions.
Hence, the present inventors introduced a small-sized electric barrel galvanizing apparatus, prepared an alkaline zincate zinc plating bath, and tried to investigate the reason why rust inhibitive performance differs when a surface treatment agent is applied to galvanized bolts and screws. As a result, some cause-and-effect relationships were grasped, although the reason for the difference in rust inhibitive performance could not be clarified because of the very complex phenomena in a galvanizing bath to which a brightener is added. For example, it became apparent that when pickling which involves washing with diluted nitric acid, which is usually performed before chromate treatment, is performed, the rust inhibitive performance is impaired.
Also, it became apparent that when the current density of galvanizing is increased to complete galvanizing in a short time, the rust inhibitive performance deteriorates when a surface treatment agent is applied.
Furthermore, it became apparent that when small bolts or screws having an outside diameter of a threaded portion of not more than 3 mm are galvanized by use of a barrel plating apparatus, an expected level of rust inhibitive performance when a surface treatment agent is applied cannot be obtained to such small bolts or screws even when galvanizing is performed with an appropriate current density which is found beforehand.